(1) Field of the Invention
The present invention relates to an image forming apparatus that can switch the temperature of a fixing member during fixing to a different value.
(2) Description of Related Art
In recent years, there has been increasing demand for power-saving electrophotographic image forming apparatuses. To meet the demand, technology for reducing power consumption of a fixing device included in an image forming apparatus by switching a fixing temperature according to the amount of toner to be fixed has been proposed.
For example, Japanese Patent Application Publication No. 10-288911 (hereinafter, referred to as “Patent Literature 1”) discloses technology of counting the number of black dots in a predetermined number of lines in image data targeted for printing, and, when the counted number is smaller than a predetermined value, setting the temperature of a heating roller of the fixing device to a temperature lower than a normal temperature to save power.
This is because the amount of toner per unit area of an image having few black dots, which is typically a text image, is small, and fixing can be performed at a temperature reduced within a predetermined range. Furthermore, small deterioration of an image quality, if any, is hardly noticeable in the text image.
In recent years, there has been an increase in the number of cases where an image targeted for printing includes an image of given coded identification information (hereinafter, referred to as an “identifier image”), such as a QR code (a registered trademark of Denso Wave Incorporated, hereinafter the same applies) and a bar code. In such cases, switching of the fixing temperature using the technology disclosed in Patent Literature 1 described above might cause any inconvenience.
That is, since an identifier image is typically placed at a position in a margin at an end of a recording sheet in terms of layout, adoption of a “control method using the number of dots as a criterion of judgment” as disclosed in Patent Literature 1 described above is likely to cause the number of black dots to be judged to be smaller than a predetermined value. As a result, the fixing temperature is set to a low temperature, and the image quality is likely to be deteriorated due to poor fixing performed at the low temperature.
Users can recognize and understand text images and photo images included in printed materials even when an image quality thereof is deteriorated to some extent. Identifier images, however, cannot be recognized by users as they stand, and are required to be read by a code reader and the like, and decoded. When an image quality of an identifier image is deteriorated, it becomes difficult to accurately read and decode the identifier image and to acquire needed information. This makes the identifier image meaningless.
The present invention aims to provide an image forming apparatus that can form images without causing deterioration of the image quality of identifier images, while saving power by appropriately switching the fixing temperature.